Development of A Gadolinium-doped Water Cherenkov Detector for The Observation of Supernova Relic NeutrinosDevelopment of A Gadolinium-doped Water Cherenkov Detector for The Observation of Supernova Relic Neutrinos超新星背景ニュートリノ探索のためのガドリニウム添加水チェレンコフ検出器の研究